Hydraulic fracturing requires the use of well treating materials capable of enhancing the production of fluids and natural gas from low permeability formations. In a typical hydraulic fracturing treatment, fracturing treatment fluid containing a solid proppant is injected into the formation at a pressure sufficiently high enough to cause the formation or enlargement of fractures in the reservoir. The proppant is deposited in the fracture, where it remains after the treatment is completed. After deposition, the proppant serves to hold the fracture open, thereby enhancing the ability of fluids or natural gas to migrate from the formation to the wellbore through the fracture.
Many different materials have been used as proppants including sand, glass beads, walnut hulls, and metal shot as well as resin-coated sands, intermediate strength ceramics, and sintered bauxite; each employed for their ability to cost effectively withstand the respective reservoir closure stress environment. The relative strength of these various materials increases with their corresponding apparent specific gravity (ASG), typically ranging from 2.65 for sands to 3.4 for sintered bauxite. Unfortunately, increasing ASG leads directly to increasing degree of difficulty with proppant transport and reduced propped fracture volume, thereby reducing fracture conductivity.
Another common problem in fracturing operations is the loss of fracturing fluid into the porous matrix of the formation. The loss of fracturing fluid into the formation has an effect on the fracture size and geometry created during the operation. The efficiency of fracturing fluids containing conventional proppants is enhanced (or fluid loss is mitigated) by thickening of the fracturing fluid. Thickening of the fracturing fluid enables it to carry proppant into the fracture.
Bridging solids are typically used to increase the viscosity of the fracturing fluid. Bridging solids are typically either viscosifying polymers, such as insoluble starches, or sized particulate solids such as silica flour or calcium carbonate.
The viscosity of the fracturing fluid filtrate leaked into the rock matrix affects the rate of fluid loss since increased viscosity of the fluid flowing within the matrix creates resistance. In conventional polymer-based fracturing fluids, this effect is typically ignored since the bridging solids employed are too large to penetrate the rock matrix. Such solids are typically filtered at the rock face, thereby creating a relatively impermeable filter cake. This, in turn, limits the loss of filtrate to the formation. As such, the filtrate penetrating the matrix is non-viscosified brine (typically 2% KCl), having a viscosity similar to that of the formation fluid. No additional resistance to movement within the matrix is needed. Unfortunately, filter cakes are often damaging to the desired conductivity of the proppant pack; in some cases reducing the proppant pack conductivity by over 90%.
More recently, attention has been drawn to the use of ULW materials as proppant materials. ULW materials are often desirable since they reduce the fluid velocity required to maintain proppant transport within the fracture. This, in turn, provides for a greater amount of the created fracture area to be propped open. Such ULW proppants, like conventional heavier proppants, have the capability to effectively withstand reservoir closure stress environments while increasing fracture conductivity.
Utilization of ULW proppants in slickwater fluids has gained favor in reservoirs having permeabilities between 0.001 to 1.0 mD since bridging solids are not needed for viscosification or proppant transport and fluid loss in such tight formations is not significant. Slickwater fluids are basically fresh water or brine having sufficient friction reducing agent to minimize tubular friction pressures. Such fluids have viscosities generally only slightly higher than unadulterated fresh water or brine. Since such slickwater fluids do not contain bridging solids, they do not build a filtercake and thus are considered inherently non-damaging to the proppant pack.
It is desirable to expand the application of slickwater based fluids incorporating ULW proppants in reservoirs having permeabilities greater than 1.0 mD. The use of slickwater fluids containing ULW proppants in higher permeability reservoirs is highly desirable since slickwater is substantially non-damaging to the formation face. In such higher permeability reservoirs, however, it is important that fluid efficiency of slickwater fluids not become compromised since sufficient fluid must remain in the fracture to facilitate continued propagation of the fracture and maintain adequate width for proppant movement. Further, such fluids must be capable of viscosifying without the buildup of filter cake.